Air conditioning systems for the passenger compartments of motor vehicles are well known. Such systems include arrangements for the flow of air across the internal surface of the windscreen of the motor vehicle. In general, these systems comprise an inside heat exchanger (located within the passenger compartment) and an outside heat exchanger (located outside the passenger compartment). A pair of fluid passages connect the heat exchangers to allow the circulation of fluid through the heat exchangers. An expansion device is positioned in one of the fluid passages. A compressor and accumulator/dryer is positioned in the other fluid passage. When fluid is pumped by the compressor through the outside heat exchanger, the expansion device, the inside heat exchanger and the accumulator/dryer in succession, air passing through the inside heat exchanger is cooled as the air flows into the passenger compartment across the windscreen. When fluid is pumped in the reverse direction through the inside heat exchanger, the expansion device, the outside heat exchanger and the accumulator/dryer in succession, air passing through the inside heat exchanger is heated as the air flows into the passenger compartment across the windscreen. A reversing valve can be positioned in the other fluid passage to provide the required flow direction for the fluid.
During the cooling cycle, the air passing through the inside heat exchanger may be de-humidified, leading to a build-up of condensation on the inside heat exchanger. During a subsequent heating cycle, the condensation may be evaporated, increasing the risk of internal fogging of the windscreen. This potential problem may be avoided either by limiting the time of the heating cycle, or by using a second inside heat exchanger during the heating cycle (with the first inside heat exchanger only being used during a cooling cycle). Both of these potential solutions have limitations.